postcss maps are using toJSON behavior

Seems I was wrong about the expected type to be string, turns out the returned map is an internal map object that [implements toJSON behavior](https://github.com/mozilla/source-map/blob/master/lib/source-map/source-map-generator.js#L370) which should be called to get the actual source map object.

This reverts 22d399e1b08caf6f8c1ba3a2b8bf6bf9b2b0f531 of #1525